Bidirectional pulse generator for video disc stylus deflector transducer

ABSTRACT

A bidirectional pulse generator suitable for driving a video disc stylus deflector transducer. A differential amplifier is arranged with both its inverting and non-inverting input terminals connected for receiving a prescribed pulse waveform. First and second diodes are respectively connected between the inverting and non-inverting input terminals and a point of bilevel potential for selectively conditioning one or the other of the diodes to conduct, thereby establishing the quiescent output potential of the amplifier and concurrently shunting the pulsed waveform from one or the other of the amplifier input terminals.

This is a continuation of Ser. No. 098,243, filed Nov. 28, 1979.

This invention relates to pulse generators and in particular to acircuit for producing bidirectional pulses. Bidirectional pulsegenerators of the type described herein have been found useful fordriving a video disc stylus deflector transducer.

Certain video disc player systems employ a track following signal pickupstylus for recovering prerecorded information from information-bearingtracks disposed on a record disc. The stylus is secured to a stylus armwhich is compliantly mounted in an arm carriage assembly for translatingthe stylus/stylus arm radially across the disc in synchronism with therate of advance of the information due to rotation of the disc. A stylusdeflector is incorporated with the carriage and stylus arm to correctfor anomalous radial stylus movement (inward or outward) or to producespecial effects such as stop motion, slow motion, record preview, etc.

A typical deflector transducer comprises a permanent magnet secured tothe stylus arm disposed between two electromagnetic coils secured to thecarriage assembly. Energizing the coils produces a magnetic fieldtherebetween to induce a relatively precise translation of the permanentmagnet and thereby a radial translation of the stylus/stylus arm. Seefor example U.S. Pat. No. 4,262,174, filed May 15, 1979, by J. C.Bleazey and entitled, "Track Skipper for Video Disc" assigned to thecommon assignee with this invention.

The present invention is a circuit suitable for producing energizingpulses for the deflector coils capable of providing stylus translationsor deflections of varying distance in both the inward and outward radialdirection.

A waveform generator responsive to a first control signal generates asignal having a prescribed waveshape, the amplitude of which beingproportional to the duration of the first control signal. Signal fromthe waveform generator is selectively applied to either the inverting ornon-inverting input terminal of a differential amplifier depending uponthe desired polarity of drive signal. The amplifier input terminalsignal selection is controlled by a second control signal whchconcurrently establishes the quiescent bias potentials of the amplifierinput terminals and thereby the amplifier output quiescent potential.For example, if a negative-going drive pulse is desired the secondcontrol signal conditions the amplifier non-inverting input terminalpotential bias at a relatively high potential and conditions theinverting input terminal to receive signal from the waveform generator.The amplifier quiescent output potential assumes a relatively highpotential responsive to such bias and upon the occurrence of a firstcontrol signal pulse the signal applied by the waveform generator to theinverting input terminal is amplified and inverted at the amplifieroutput. The amplifier output potential goes through a negative excursionfrom the relatively high quiescent potential tending toward therelatively low supply potential. Conversely, if a positive going outputdrive signal is desired, the second control signal conditions the biasat the non-inverting input terminal relatively low (and thereby theamplifier output quiescent potential low) and conditions thenon-inverting input terminal to receive signal from the waveformgenerator. In this case the output drive signal excursion is positivegoing, from the relatively low quiescent output potential toward therelatively high supply potential.

In the drawings:

FIGS. 1 and 2 are circuit schematic diagrams of bidirectional pulsegenerators embodying the present invention; and

FIG. 3, consisting of a-e, is a time versus voltage waveform diagramrelating the output potential of the FIG. 2 circuit to the appliedcontrol voltages.

In FIG. 1 a transducer 22, e.g., an electromagnetic coil, piezoelectriccrystal, etc., is coupled via capacitor 23 to the output connection 13of the bidirectional pulse generator 100. Capacitor 23 serves thepurpose of isolating the DC potential at connection 13 from thetransducer 22 so that positive-going signals and negative-going signalswith prescribed minimum frequency components are coupled to thetransducer having an average DC potential about reference potential 20.

The pulse generator 100 is responsive to two control signals, adeflection pulse applied to terminal 18 and a direction signal appliedto terminal 19. In normal operation, transitions of the two controlsignals do not occur simultaneously but rather the state of thedirection signal is established with the pulse generator responsethereto completed before the occurrence of a deflection pulse.

In the FIG. 1 circuit a differential amplifier 10 has a non-invertinginput connection 15 connected to the output terminal 16 of waveformgenerator 11 by a resistor R4 and connected to reference potential 20 byresistor R5. An inverting input connection 14 of the differentialamplifier 10 is connected to waveform generator 11 by the seriallyconnected resistors R2 and R3 and is connected to the output connection13 by feedback resistor R1. A first diode D1 connects the intersectionof resistors R2 and R3 to the direction signal buffer circuit 12 and asecond diode D2 connects the non-inverting input connection 15 ofamplifier 10 to circuit 12. The output impedance of both the waveformgenerator 11 and the direction signal buffer circuit 12 are relativelylow compared to the resistances of R3 and R4.

In operation, the direction signal applied to terminal 19 is one of twolevels, high or low. These levels are buffered by circuit 12 to producea signal which is applied to the interconnection 21 of diodes D1 and D2.In the presence of a relatively high potential at connection 21, diodeD2 is conditioned to conduct thereby imposing the relatively highpotential at connection 21 on the non-inverting input connection 15(less a forward diode drop). Concurrently diode D1 is reverse biased orcut off effectively eliminating it from the circuit. In the absence of adeflection pulse, terminal 16 is substantially at reference potential,configuring the circuit as a non-inverting amplifier with a relativelyhigh DC potential signal at its input, and exhibiting a DC gain at itsoutput terminal 13 of (1+R1/(R2+R3)) producing a relatively high outputpotential where R1, R2 and R3 are the respective resistance values ofthe corresponding resistors.

Application of a deflection pulse to terminal 18 conditions the waveformgenerator 11 to output a positive-going pulse of prescribed amplitudeand waveshape to terminal 16, which pulse is coupled by resistors R2 andR3 to the inverting input connection 14 of amplifier 10. The pulseapplied to terminal 16 is precluded from modulating the non-invertinginput connection 15 by the clamping action of forward-biased diode D2.The amplifier 10 inverts the sense of the pulse applied to its invertinginput. The pulse is propagated through the amplifier appearing at theoutput connection 13 inverted and modified by the gain factorR1/(R2+R3). The output pulse at connection 13 commences at the relativehigh DC level established by the direction signal and tends toward theopposite or relatively low supply potential.

On the other hand, if a low direction potential is applied to terminal19, a relatively low potential appears on interconnection 21, whichpotential is insufficient to condition diode D2 to conduct. Thepotential at connection 15 is pulled near reference potential, e.g.,ground, by the conduction path provided by resistor R5.

The potential offset of the amplifier is arranged so that under thesecircumstances the output potential, though relatively low, whentranslated back by the resistors R1 and R2 is sufficient to conditiondiode D1 to conduct. Now upon the occurrence of a deflection pulse, suchpulse is precluded from modulating the inverting input connection 14 bythe clamping action of diode D1, but is coupled to the non-invertinginput connection 15 by resistor R4. The pulse is propagated to theoutput non-inverted, and modified by the gain factor (1+R1/R2). Theoutput pulse at connection 13 commences at the relatively low DCpotential established by the low direction signal and goes through anexcursion tending toward the positive or relatively high supplypotential.

In FIG. 2, amplifier 30 driving complementary transistor emitterfollowers Q1 and Q2, forms a composite differential amplifier 60 withnegative feedback provided by resistor R10. The output signal atconnection 40 from amplifier 60 is coupled via capacitor C2 to atransducer 22. A direction signal buffer circuit 12', comprisingtransistor Q4 and resistors R14 and R18 is responsive to a bileveldirection signal applied to terminal 49 to generate a potential forselectively conditioning diodes D1 and D2 to conduct. A low potentialsignal applied to terminal 49 conditions Q4 to be non-conducting so thatthe potential at interconnection 41 is determined by the ratioR14/(R14+R18) times the value of the supply potential V. Under theseconditions, the potential at interconnection 41 is relatively high whereR14 and R18 are the respective resistance values of the correspondingresistors. On the other hand a high signal applied to terminal 49conditions transistor Q4 to conduct essentially clamping interconnection41 to ground potential, i.e., a relatively low potential. The dynamicimpedance looking into the buffer circuit 12' from interconnection 41 isthe saturation resistance of Q4 when conducting, or the impedance of theparallel connection of resistors R18 and R14, i.e. R18·R14/(R18+R14),the latter impedance being substantially smaller than the resistance ofresistor R13. As in the circuit of FIG. 1, application of DC potentialsto the interconnection of the diodes selectively establishes the DC biasof the amplifier and determines which of the inverting and non-invertingamplifier input terminals receives signal applied by waveform generator11'.

Waveform generator 11' comprises transistor Q3, resistors R17, R19 andcapacitor C1. The resistance of R19 is small compared to the resistanceof R17 and is included to improve the fall time at terminal 42 and tolimit dissipation in transistor Q3 when it conducts initially todischarge capacitor C1. The potential at input terminal 48 is normallysufficiently high to condition transistor Q3 to be conducting insaturation, thereby grounding terminal 42. The potential at connection43 discharges through resistor R19 to a value V1 equal to R19/(R17+R19)times V supply. The occurrence of a deflection pulse at terminal 48turns transistor Q3 off, terminal 42 becomes a relatively high impedancepoint thereby allowing connection 43 to charge toward V supply throughresistor R17 with a time constant τ=R17C1. Such charging produces afairly linear positive going ramp potential signal for a relativelylarge portion of the potential excursion from potential V1 to V supply.The potential V43 at connection 43 is approximated by:

    V43=V supply (1-exp(-t/R17C1))                             (1)

where R17 and C1 are the respective resistance and capacitance values ofthe corresponding devices and from which it can be seen that the longerthe duration of time t the larger the amplitude of potential V43. Thetime t corresponds to the duration of the deflection pulse, i.e., thetime Q3 is non-conducting.

The potential V43 is coupled via resistor R19 to the terminal 42 fromwhich it is applied to the non-inverting terminal 46 or invertingterminal 45 of amplifier 30 depending on which diode is conducting.

The resistors R16 and R15 form a potential divider to bias connection 46slightly positive for diode D2 non-conducting. This potential is chosensuch that when amplified by amplifier 60 and fed back to connection 44via resistors R10 and R11 it conditions diode D1 to conduct. Thus forconnection 46 at a relatively low potential, diode D1 is forward biasedand clamps the input signal to the inverting signal terminal 45 at a DCpotential.

Referring to FIG. 3, waveform 3(a) corresponds to the deflection pulseapplied to terminal 48, and waveform 3(b) is the potential produced atterminal 42. At time To, transistor Q3 is conducting and terminal 42 isclamped to ground. At time T1 the deflection pulses goes negativeturning off transistor Q3 and allowing connection 43 and therebyterminal 42 to charge toward the positive supply potential, the longerthe charging time the higher the resulting potential, as demonstrated bythe broken lines in the figure. Finally, at time T2 the deflection pulseterminates, transistor Q3 again conducts and terminal 42 is clamped toground. The potential at terminal 42 is applied to the amplifiernon-inverting input terminal 46 with a positive-going signal appearingat output connection 40 (waveform 3(d)) which is coupled by capacitor C2to transducer 22 (waveform 3(e)).

Waveform 3(c) represents the direction signal applied to terminal 49. Attime To the direction signal is high, transistor Q4 conducts and node 41is low with the consequence of biasing the ouput at connection 40 low.Between times T4 and T5 the direction pulse is going through atransition from high toward low, turning Q4 off, and biasing connection46 relatively high, etc. From waveform 3(d) it is seen that thesubsequent deflection signal at node 42 is inverted (time T6-T7). Thetransition (T4-T5) of the direction signal is relatively long comparedwith the signal transition (T6-T7) to prevent the DC potentialtransition at connection 40 from being coupled in sufficient amplitudeby capacitor C2 to energize the transducer 22.

Table 1 is a list of device values for a particular realization of theFIG. 2 circuit.

                  TABLE 1                                                         ______________________________________                                        Resistor       Resistance                                                     ______________________________________                                        R10            100K Ohm                                                       R11             15K Ohm                                                       R12             51K Ohm                                                       R13            330K Ohm                                                       R14             10K Ohm                                                       R15            110K Ohm                                                       R16            1.3M Ohm                                                       R17             15K Ohm                                                       R18             12K Ohm                                                       R19             1K Ohm                                                        Capacitor      Capacitance                                                    Cl             0.1μ Farad                                                  Diode D1, D2   IN914                                                          Transistors Q3, Q4                                                                           MPSA17             Motorola                                    Transistor Q1  MPSU01             transistors                                 Transistor Q2  MPSU51                                                         Amplifier 30   CA324                                                          ______________________________________                                    

What is claimed is:
 1. A bidirectional pulse generator for driving avideo disc stylus transducer, said pulse generator energized by a supplypotential and being capable of producing positive going and negativegoing drive pulses with respective amplitudes substantially equal tosaid supply potential comprising:respective terminals for applying firstand second control signals; an output terminal; a waveform generatorhaving an output connection, said generator being responsive to saidfirst control signal for generating an output signal having an amplitudeproportional to the duration of said control signal; an amplifier havinga signal input terminal connected for receiving said output signal andhaving an output connection; circuit means responsive to said secondcontrol signal for selectively establishing the operating mode of theamplifier between one of an inverting and non-inverting mode and forselectively establishing a quiescent potential at the output connectionof said amplifier at one of a relatively high and a relatively lowpotential, said relatively high and low potentials tending toward therelatively positive and negative potential of said supply potential; anda capacitor connected between said amplifier output connection and saidoutput terminal.
 2. The bidirectional pulse generator set forth in claim1 wherein the waveform generator comprises:a further capacitor having afirst plate connected to reference potential and having a second plate;means for charging the second plate of said further capacitor to aprescribed potential; and means responsive to said first control signalfor selectively discharging said second capacitor plate and clamping thepotential thereat at a prescribed amplitude.
 3. The bidirectional pulsegenerator set forth in claim 1 wherein the amplifier comprises:adifferential amplifier having an inverting and a non-inverting signalinput terminal and an output terminal; and respective means connectingthe differential amplifier inverting input terminal and non-invertinginput terminal to the output terminal of the waveform generator.
 4. Thebidirectional pulse generator set forth in claim 3 wherein the circuitmeans comprises:means responsive to said second control signal forgenerating at an output connection thereof a bistate bias signal ofrelatively high or relatively low bias potential; means including firstand second unidirectional conduction devices having respective first andsecond ends, said first and second unidirectional conduction devicesinterconnected in serial arrangement so that both devices in said serialarrangement are poled to conduct in the same direction and whereinrespective ends of said serial arrangement are connected to saidamplifier inverting and non-inverting input terminals; means forconnecting the output connection of the bistate bias signal generatingmeans to the interconnection of the first and second unidirectionalconduction devices, said bistate bias signal alternatively biasing saidfirst and second unidirectional conduction devices into conduction. 5.The bidirectional pulse generator set forth in claim 3 wherein saidcircuit means comprises:means for diverting the waveform generatorsignal from the amplifier non-inverting input terminal concurrent withthe non-inverting input terminal being biased at said relatively highpotential and for diverting the waveform generator signal from theinverting input terminal concurrent with the non-inverting inputterminal being biased at said relatively low potential.
 6. Circuitry forenergizing a video disc stylus deflector comprising:a waveform generatorresponsive to a first control signal for generating an output signal ofprescribed amplitude, said generator having an output terminal;amplifier means having inverting and non-inverting input terminals andan output terminal; a first resistor connected between the waveformgenerator output terminal and the amplifier non-inverting inputterminal; a second resistor connected between the amplifiernon-inverting input terminal and a point of relatively low referencepotential; third and fourth resistors having respective first endsconnected to the waveform generator output terminal and the amplifierinverting input terminal respectively, and having respective second endsconnected to a first node; a fifth resistor connected between theamplifier inverting input and output terminals, said fifth and fourthresistors determining the non-inverting gain factor of the amplifier andsaid fifth, fourth and third resistors establishing the inverting gainfactor of the amplifier; a first unidirectional conduction device havinga first end connected to said first node and having a second endconnected to a second node; a second unidirectional conduction devicehaving a first end connected to said second node and a second endconnected to the amplifier non-inverting input terminal; means forselectively applying first and second potentials to said second nodewhereby one of said first and second unidirectional conduction devicesis conditioned to conduct in the presence of one of said first andsecond potentials and the other of said first and second unidirectionalconduction devices is conditioned to conduct upon application of theother of said first and second potentials; and means connecting theamplifier output terminal for energizing said stylus deflector.